The proposed research will examine the sites and mechanism of organophosphate toxicity at discrete loci in the central nervous system, in particular the ventral surface of the medulla oblongata. Organophosphates are known to act as potent inhibitors of acetylcholinesterase, the enzyme responsible for hydrolysis of acetylcholine, and produce effects nearly equivalent with excessive cholinergic stimulation. When applied to the ventral medulla, organophosphates elicit a profound, long-lasting vasodepression which is reversed in the presence of the muscarinic antagonist atropine and oxime reactivators of the enzyme. These observations implicate the presence of neuroanatomically well-defined sites on the ventral medulla and a mechanistic role for central cholinergic tracts in maintaining cardiovascular function. We hope to identify cholinergic nerve tracts in the ventral medulla by employing fluorescent phosphonates which not only induce a vasodepressor response but also react with acetylcholinesterase to label distinct sites, hence rendering the neurons visible in fluorescence microscopy. The role of oxime reactivators in reversing the vasodepression and their capacity for distribution in the central nervous system will be compared with the extent of enzyme reactivation, thus allowing estimation of their mechanism in bringing about (putative) therapeutic benefit. Stereospecificity in the active site of acetylcholinesterase and optical relationship with that of the triesterases present in mammalian sera will be investigated employing enzymatic resolution of the enantiomeric fluorescent phosphonates.